The regulation of liquid water level in a boiling water boiler is a very old and highly developed art. Hundreds of thousands of these systems exist, and in every one of them the maintenance of a correct liquid water level is a critical matter. Absence of sufficient liquid water can lead to serious damage and occasionally to violent explosions. Very detailed standards are written for them, many of them being specified by insurance carriers who insure these systems.
Any adequate system willinclude means to sense a too-low level, and shut off the burners until a suitable level is restored. Also they include means to add water to restore the boiler water to a safe liquid level, and then permit the boiler system to return to operation.
In view of what would seem to be a simplistic situation amenable to little more than an off-on switch for control, it is surprising that there is a perceived need to make the safety standards more rigorous, and that it requires inventive efforts to attain these objectives, yet such is the present situation.
If it were only a matter of simply maintaining the liquid water level at or above some lower level, with the boiler in continuous operation and all systems functioning correctly, there would be no perceptible problem. But still malfunctions and explosions do occur. It is an object of this invention to provide a liquid water level control system which is sensitive not only to correct and incorrect liquid water levels, but which is also sensitive to, and reactive to, control system malfunctions.
The term "liquid level" is used herein to describe the interface between the liquid phase and the next phase above it. This next phase will usually be a gas, for example steam, but it might instead be a foam. What is important to this invention is the fact that the electrical conductivity of the liquid will be many times greater than that of the gas, vapor, or foam phase above it. The objective is, of course, to maintain an adequate supply of liquid such that the reservoir does not get dangerously low and such that there is always sufficient liquid to supply the necessary other phase or phases, such as steam for the radiators. For convenience in disclosure, water will be used as the example in the following specification, because it is the most important liquid commercially. However, any conductive liquid may be considered equivalent to it.
While this invention is useful with any type of boiler that utilizes a conductive liquid, especially water, its greatest utility is in the rather elementary boiler systems that are used for steam heating systems in residences and office buildings.
Such systems generally use a boiler which is frequently made of cast iron, that is fired on its outer surfaces by flames from coal, gas or heating oil. These boilers are typically unattended in the sense that there is no full time boiler operator. Almost universally they are operated intermittently, because heat is provided only during working and waking hours. Late at night these systems may be shut down, and are re-started the next morning, usually under time clock control. Such supervision as exists is generally exercised by a building superintendent. These persons are not often admired for their grasp of technology, and many of them can be counted on to disable safety circuitry which might require them to get up in a very cold early morning.
It is another object of this invention to provide a liquid level control system which is unlikely to give false alarms, and which will not permit a shut-down system to restart except in the course of an approved and safe series of events. It will shut down the burners and prohibit automatic restart when certain events occur in an incorrect sequence respective to a control system malfunction.
It is therefore an object of this invention to provide control circuitry for boiler systems which provides optimum protection against boiler malfunctions.